I’ve made some for my A22 using black fire retardant potting compound, they come out almost as hard as Bakelite, and they look like Bakelite too!
this is the stuff I used Hope it’s ok to post a link to it! It’s not cheap, but so far, it’s turned out pretty good, the knobs don’t seem to have suffered any shrinkage. It does take a while to get fully hardened, and if you mix it slightly wrong like I did, then it takes even longer, with the stuff feeling like soft plastic for a long while.
Here’s a link to my A22 restoration, page 3 shows the making of the knobs! All very experimental...
Regards
Lloyd